A solid-state imaging device is generally so constituted as shown in FIG. 7 and has a potential profile as shown in FIG. 8.
As shown in FIG. 7, a solid-state imaging device is formed upon an n-type silicon substrate 1. A p-type well 2 is formed on the surface of the substrate 1. This p-type well is grounded. A photosensor region is formed of an n-type diffusion region in the p-type well 2. A p+-type hole accumulation layer 4 is formed on the surface of each photosensor region 3. An gate insulation film 5 is formed on the surface of the substrate 1. A first polysilicon transfer electrodes 6 are provided on the film 5. A second polysilicon transfer electrodes 7 are provided above the transfer electrodes 6. An insulation film 8 covers the surface of transfer electrodes 6 & 7, the surface of the photosensor region 3 and the periphery thereof. The insulation film 8 is termed PSG film since it is usually composed of PSG (Phospho-Silicate Glass), however, other materials such as BPSG (Boro-Phospho Silicate Glass) may also be suitable. An aluminum light shielding film 9 is formed on top of the insulation film 8. A channel region 10 of a vertical register is formed in the p-type well 2 below the transfer electrodes 6 & 7.
Signal charge packets are generated in the photosensor region 3. These signal charge packets are read-out to the channel region 10 by applying a positive voltage to the second transfer electrode 7. Subsequently, the signal charge packets are transferred in the channel region 10 to the vertical direction by applying a square wave alternating between negative voltage and zero voltage to the first transfer electrodes 6. A similar square wave with opposite polarity is simultaneously applied to the second transfer electrodes 7.
The hole accumulation layer 4 is at ground voltage since it is electrically connected to the grounded p-type well 2. The layer 4 serves to absorb the electrons which are prone to become dark current. The dark current cause a spurious signal. The reason for providing such hole accumulate layer 4 will be described below.
In the insulation film 8, there are fixed charges and mobile charges of sodium ions. When electrons accumulate on the surface of the photosensor region by such fixed charges and mobile charges, it causes an increase of dark current. To avert such a phenomenon, a p+-type semiconductor layer where major carriers are positive holes, i.e., the hole accumulation layer 4, is formed on the surface of the photosensor region 3. This permits the electrons to recombine with the positive holes which are the major carriers.
Therefore, in a solid-state imaging device of a type having such hole accumulation layer 4, it is possible to reduce the dark current in comparison with any solid-state imaging device without such hole accumulation layer 4.
In conventional solid-state imaging devices, however, the hole accumulation layer 4 at ground voltage is prone to shift toward the positive by said fixed charges or when, for example, the voltage of the transfer electrode 6 or 7 is made positive. The hole accumulation layer 4 causes a depletion upon occurrence of the shift toward the positive. If such depletion is induced, large dark current is generated.
The dark current is reducible by forming the insulation film 8 thicker. This is because the field effect exerted on the photosensor region 3 by the fixed charges existing on the surface of the insulation film 8 diminishes in accordance with an increase of the thickness of the insulation film 8. Consequently, the dark current can be reduced by increasing the thickness of the insulation film 8. As shown in FIG. 9, however, if the insulation film 8 is made thicker as indicated by a solid line (a), the opening to oblique incident light is rendered wider compared with the thinner insulation film 8 as indicated by a broken line (b). Eventually, an undesired smear component is caused by direct incidence of the oblique light to the channel region 10 of the vertical register. (Oblique incident light as indicated by a solid arrow (c) comes into the channel region 10 directly along with oblique incident light as indicated by a broken arrow (d).) This phenomenon restricts the acceptable thickness of the insulation film 8. It follows therefore that it is desirable to reduce the dark current without increasing the thickness of the insulation film 8.